1 Field of the Invention
The present invention generally relates to a vehicle brake control system. More particularly, the present invention relates to a vehicle brake control system that cooperatively controls a frictional braking system and a regenerative braking system of a vehicle during an initial stage of braking.
2. Background Information
A cooperative control apparatus for a vehicle brake system is typically configured to perform cooperative control of a frictional braking system and regenerative braking system to provide a target braking torque in response to a braking operation or other vehicle driving state. As appreciated in the art, a vehicle brake system that includes a regenerative braking system and a frictional braking system can be referred to as a combination braking system or simply a combination system. During this cooperative control operation, regenerative braking is preferentially used, especially during the initial state of the braking since the target braking torque is generally small. However, if the target braking torque is increased by an increase in the force of the braking operation, the regenerative braking may not be sufficient to achieve the target braking torque.
When the target braking torque can no longer be supplied solely by regenerative braking due to such factors as a subsequent increase in the target braking torque by an increased braking operation force, a change occurs from a state in which braking is performed solely by regenerative braking to a state in which braking is performed by regenerative braking as well as frictional braking. In this event, the frictional braking is used with the regenerative braking to compensate for the shortfall in braking torque. Accordingly, regenerative braking is used as much as possible, and the use of frictional braking can be kept to a minimum. Therefore, the amount of the kinetic energy of the vehicle that can be recovered as electrical energy by regenerative braking can be maximized while the loss of vehicle kinetic energy as heat due to frictional braking can be minimized. As a result, energy efficiency of the vehicle can be enhanced, which can reduce fuel consumption or electrical consumption by the vehicle.
As understood in the art, a hydraulic frictional braking system typically includes a master cylinder containing one or more pistons, and a brake fluid reservoir. The master cylinder communicates with the brake fluid reservoir via at least one fluid port. When a brake pedal, for example, is depressed, a piston in the master cylinder is moved by a push stroke distance past the fluid port to generate fluid pressure in the brake lines. This fluid pressure operates the frictional brakes to close on the wheel rotor, for example. However, the fluid pressure is typically not generated to an amount sufficient to operate the frictional brakes as desired until after the piston blocks the fluid port. Hence, a delay period may exist between the time that the brake pedal is depressed and the time that the sufficient fluid pressure is generated. This delay period may cause a response delay in the frictional braking operation that tends to be large in comparison with any response delay in the regenerative braking operation. Therefore, when the change occurs from the state of braking solely by regenerative braking during the initial stage of the braking operation to the state of regenerative braking that is supplemented by frictional braking, the frictional braking force may not be applied as rapidly as the regenerative braking force due to the response delay in the frictional braking operation.
Japanese Unexamined Patent Application Publication No. 2007-196924 describes a system that attempts to reduce the delay in the application of a frictional braking force. Specifically, that system applies both a regenerative braking force and a frictional braking force during the initial stage of braking, instead of only applying regenerative braking to provide the target braking torque. Therefore, even when the application of the frictional braking force is delayed, the higher response application of the regenerative braking force can compensate for such a delay.